The harmful chemical, dubbed trichlorofluoromethane (CFC-11), has been banned for decades in an effort to save the ozone layer. However, a new study led by the U.S. National Oceanic and Atmospheric Administration (NOAA) shows an unknown source has been releasing large amounts of CFC-11 into the atmosphere since 2013.

“We’re raising a flag to the global community to say, ‘This is what’s going on, and it is taking us away from timely recovery of the ozone layer,’” said Stephen Montzka, lead author of the study and a scientist at NOAA.

The ozone layer protects Earth from all kinds of nasty radiation emitted by the sun, including ultraviolet light. However, some of that radiation started slipping through late last century, when greenhouse gases created a hole in the ozone layer over Antarctica.

The hole in the ozone layer forms each September over Antarctica, and while it has been shrinking, last year it was still approximately 20 million square kilometres across. That’s down from its average size of 26 million square kilometres.

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However, Montzka says that progress could be erased if the mystery polluter is not identified and stopped soon.

The unreported source of CFC-11 appears to be somewhere in east Asia, according to Montzka and his co-authors from the U.S., the U.K. and the Netherlands. They say the emission levels have raised the overall concentration of CFC-11 in the atmosphere to levels not seen since the late 1990s.

“Further work is needed to figure out exactly why emissions of CFC-11 are increasing, and if something can be done about it soon,” Montzka said in a statement.

“We don’t know why they might be doing that and if it is being made for some specific purpose, or inadvertently as a side product of some other chemical process.”

FRIDGE-CHILLER, OZONE KILLER

CFC-11 belongs to a family of chemicals called chlorofluorocarbons, which break down in Earth’s stratosphere and destroy any ozone molecules they come in contact with.

CFC-11 was a common household chemical found in aerosol spray cans, air conditioners and refrigerators through much of the 20th century, until scientists sounded the alarm about the harm it was causing in the 1970s.

A total of 148 countries heeded their warning and signed the Montreal Protocol in 1987, when they pledged to ban CFCs and cut their global emission levels to virtually zero by 2010.

Those efforts have led to an overall 15 per cent decline in the concentration of ozone-depleting substances in the air since 1994, the study says.

Nevertheless, CFC-11 remains the second-most plentiful ozone-killing gas in the atmosphere, where it lingers for about 50 years. Nature also scrubs about two per cent of the CFC-11 concentration from the atmosphere every year, but the decline is slow, experts say.

Concentrations of other banned ozone-depleting substances have continued to decline without interruption, according to the NOAA data.

NOT NATURAL

The study authors say CFC-11 emissions were close to zero from 2006-2012, and that the overall amount of the chemical in the atmosphere was steadily declining up until 2013, after which the decline started to slow down.

“The increase in emission of CFC-11 appears unrelated to past production,” they write in the study paper, which is published in the journal Nature.

“This suggests unreported new production, which is inconsistent with the Montreal Protocol agreement to phase out global CFC production by 2010.”

Montzka says he was surprised to see the CFC-11 levels suddenly level out, after years of steady decline.

“If the sources of these emissions can be identified and mitigated soon, the increased damage to the ozone layer should be minor,” Montzka wrote in a fact sheet provided by NOAA. “If not remedied soon, however, delays in the ozone recovery would be expected.”

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Small amounts of the chemical still seep into the atmosphere from destroyed buildings and old appliances, but all major sources of the gas are expected to register with the UN’s Ozone Secretariat under the Montreal Protocol.

“If these emissions continue unabated, they have the potential to slow down the recovery of the ozone layer,” the Ozone Secretariat said Wednesday, in a statement addressing the study. “It is therefore critical that we take stock of this science, identify the cause of these emissions and take necessary action.”

The Ozone Secretariat says the ozone layer is still on track to recovery by the middle of the century, but unchecked CFC-11 emissions “will put that progress at risk.”

The UN watchdog says it will put the findings to its scientific assessment panel, which includes some of the study’s authors.

“So long as scientists remain vigilant, new production or emission of ozone-depleting chemicals will not go unnoticed,” the Ozone Secretariat said.